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April 20, 2015

April 20, 2015

Posted by **mike** on Wednesday, November 30, 2011 at 1:34am.

- arithmetic -
**Reiny**, Wednesday, November 30, 2011 at 8:36amthe third term is 17 ---> a+2d = 17

a = 17-2d

sum of first 8 terms = 220

---> (8/2)(2a + 7d) = 220

4(2a + 7d) = 220

2a + 7d = 55

2(17-2d) + 7d = 55

34 - 4d + 7d = 55

3d = 21

d = 7

then a = 3

sum(6) = 3(6 + 35) = 123

check:

terms are: 3 10 17 24 31 38 45 52 ..

sum(8) = 220

sum(6) = 123

all is good

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